The present invention relates in general to field emission devices, and in particular to field emission devices comprising carbon nanotubes.
Carbon nanotubes (CNTs) have intriguing physical and chemical properties which have consequently made them the object of numerous ongoing studies (Ajayan et al., Top. Appl. Phys., vol. 80, p. 391, 2001; Dai, Acc. Chem. Res., vol. 35, p. 1035, 2002). As a result of some of these studies, carbon nanotubes have been found to be excellent cathode materials for field emission displays because of their high aspect ratio and outstanding chemical inertness (U.S. Pat. No. 5,773,921). Single-wall carbon nanotubes (SWNTs) are hollow carbon fullerene tubes that have diameters from 5 angstroms to several nanometers (nm) and can be microns (xcexcm) long or longer. Multi-wall carbon nanotubes (MWNTs) are similar, but comprise more than one concentric layer of carbon forming the tube. It has been suggested that aligned carbon nanotubes may have good field emission properties because they have higher geometric field enhancement (Wang et al., Appl. Phys. Lett, vol. 72, p. 2912, 1998). CNTs can be produced by chemical vapor deposition (CVD) (Nikolaev et al., Chem. Phys. Lett., vol. 313, p. 91, 1999; Huang et al., Appl. Phys. A, vol. 74, p. 387, 2002), arc discharge (Journet et al., Nature, vol. 388, p. 756, 1997), laser ablation (Thess et al., Science, vol. 273, p. 483, 1997), and other techniques (e.g., Derycke et al., Nano Letters, vol. 2 (10), p. 1043, 2002). Additionally, vertically-aligned CNTs can be grown on substrates possessing nanoscale metal catalysts using CVD methods (Huang et al., 2002) at temperatures from about 550xc2x0 C. to about 1200xc2x0 C.
All of the abovementioned techniques, however, have poor growth uniformity and none can practically deposit carbon nanotubes over large areas. Furthermore, the growth conditions require relatively high temperatures, which impede their utilization with low-temperature and generally inexpensive substrate materials.
Another problem with using the abovementioned CNT growth techniques for generating the cathode material for field emission displays is that the density of the CNTs produced may be too high. Researchers have found evidence that the field emission properties of high density CNT cathodes is less than expected because the neighboring nanotubes shield the extracted electric fields from each other (Bonard et al., Advanced Materials, vol. 13, p. 184, 2001). As a result, high-resolution lithography has been employed to control CNT density by creating catalytic dots capable of growing CNTs (Huang et al., Appl. Phys. A, vol. 74, p. 387, 2002). This method is very expensive, however, and requires growth on high-temperature substrates.
Thus, there is a demonstrated need to be able to harvest fabricated CNTs and apply or dispense them onto various substrate materials at low temperatures. There is also a need to be able to control the density of the CNTs in an effort to optimize their field emission properties.
The present invention is directed toward a new cathode for field emission devices, methods for making such a cathode, and methods for optimizing the electron field emission performance of such a cathode by lowering the threshold field of emission and increasing emission current. Such a cathode comprises a cathode material, which in turn comprises carbon nanotubes (CNTs) and particles. Optimization of the electron field emission performance is accomplished by modulating the density of the field emitters (CNTs) within a particulate matrix material. It is believed that the optimal concentration of CNT fibers in the cathode material mixture (CNTs and particles) is that which leaves the highest number of CNTs available for emission, but not so high that they interfere with the performance of each other through electrical shielding of the applied field. Furthermore, such a mixture can be applied to a very wide range of materials since the processing can be done at room temperature and since the optimization of CNT concentration is substrate-independent. This method is also very economical in that no high-resolution lithography processing step is required. It is likely that any application involving the use of CNT materials as field emitters could potentially benefit from this invention.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention.